sun coral dying?

[genie78]

I have a sun coral that seems to be dying. It still seems to open every night and feeds well but about 5 or 6 polyps out of the 30 or so; seem to be loosing flesh and the skeleton is coming through. What might be the most likely causes? All water tests are good as well as temp and salinity. I have some chromis, percula, and a coral beauty. Possibly nippng at it? Anyone have any ideas or thoughts I really don't want to lose this coral it is one of the better suns I've seen.
Thanks
Genie